In a spark plug used for ignition in an internal combustion engine or the like, when a voltage is applied to a center electrode and a ground electrode which are insulated from each other by an insulator, a spark occurs in a spark gap formed between a front end portion of the center electrode and a front end portion of the ground electrode (e.g., Japanese Patent Application Laid-Open (kokai) No. H11-273827).
In recent years, reduction in the diameter and size of a spark plug is desired from the standpoint of reducing the size of an internal combustion engine and the standpoint of improving the design freedom.